Abstract

Narrow potential vorticity (PV) intrusions across the tropopause (PV streamers) are interconnected with surface high- and low- pressure systems and frequently present during high-impact weather in the mid-latitudes. They often form during the breaking of Rossby waves (observed
as large meanders of the jet stream), and occur at the end of the wave life cycle. Extreme weather events are likely to increase in the future due to global warming and climate change, so, enhanced knowledge about the main infuences on these events is crucial for early warning systems.
This thesis explores the location and relationship between upper- (troughs and PV streamers) and lower- (cyclones and anticyclones) level features impacting the United Kingdom during heavy precipitation days. It then investigates the genesis of preceding Rossby waves and initiation of PV streamer formation related to heavy precipitation cases. Finally, the predictability of these features is analysed through an assessment of forecasting skill in the European Centre for Medium-Range Weather Forecasts (ECMWF deterministic models as well as the THORPEX Interactive Grand Global Ensemble (TIGGE) prediction system through the use of a novel feature-based error method.
Stratospheric PV streamers were found to be present during UK heavy precipitation cases 88% of the time in summer, 85% in autumn, 72% in spring and 63% in winter. They are
the dominant in influence on heavy precipitation for the UK in summer as well as in autumn in combination with cyclones. Cyclones are located to the north-west of the UK during all of the seasons with highest frequency in winter and least in summer. Anticyclones also have an impact
on UK precipitation by steering systems further north in the winter. The results vary regionally, with western and northern areas characterized by orographic in
influences. Eastern Scotland has the most consistent pattern of stratospheric streamer involvement, and the combination of upper-level and orographic effects create uplift of moist air leading to heavy precipitation
events. Remarkably, spring has the most variance in the distribution of upper- and lower-level features with evidence of a distinct east/west split across the country. Cyclone and anticyclone pairings dominate in the west, while stratospheric and tropospheric streamer coupling enhances precipitation in the east.
Rossby waves preceding the events are triggered from 3 to 7 days in advance, with some seasonal variations. The trigger points range from the Pacific Ocean basin, throughout North
America to the western Atlantic Ocean. The waves then proceed across the North Atlantic, where PV streamers are initiated. Triggers located to the west (or behind) the PV streamer lead to enhanced ridges and LC1 or anticyclonic streamer types, while triggers to the east (ahead of the streamer) increase the likelihood of LC2 or cyclonic streamer types. Influences to the east are the most common form of trigger closely followed by streamers forming from
recirculated stratospheric air (for example when a parcel of air re-attaches to the stratosphere).
PV streamers are generally represented well in short forecast lead times (1-2 days) with a growth in structural and location errors as lead time increases. An interesting result of the feature error method was the identification of significantly lower PV mean and maximum amplitudes (by as much as -3.5PVU) especially in the upper eastern flank of the streamer. This could be due to insfficient influx of high PV air into the streamer and would benefitt from further investigation. In associated heavy precipitation forecasts, TIGGE ensemble members with more accurate rainfall prediction have consistently better PV streamer representation than those who under-predicted the precipitation. The evidence indicates that improved understanding and prediction of PV streamers can lead to better predictability of heavy precipitation and thus
an enhancement in early warning systems.